Effects of unilateral vs. bilateral resistance training interventions on measures of strength, jump, linear and change of direction speed: a systematic review and meta-analysis

Exercises can be categorized into either unilateral or bilateral movements. Despite the topic popularity, the answer to the question as to which (unilateral or bilateral) is superior for a certain athletic performance enhancement remains unclear. To compare the effect of unilateral and bilateral resistance training interventions on measures of athletic performance. Keywords related with unilateral, bilateral and performance were used to search in the Web of Science, PubMed databases, and Google Scholar and ResearchGate™ websites. 6365 articles were initially identified, 14 met the inclusion criteria and were included in the final analysis, with overall article quality being deemed moderate. The quantitative analysis comprised 392 subjects (aged: 16 to 26 years). Sub-group analysis showed that unilateral exercise resistance training resulted in a large effect in improving unilateral jump performance compared to bilateral training (ES = 0.89 [0.52, 1.26]). In contrast, bilateral exercise resistance training showed a small effect in improving bilateral strength compared to unilateral (ES = -0.43 [-0.71, -0.14]). Non-significant differences were found in improving unilateral strength (ES = 0.26 [-0.03, 0.55]), bilateral jump performance (ES = -0.04 [-0.31, 0.23]), change of direction (COD) (ES = 0.31 [-0.01, 0.63]) and speed (ES = -0.12 [-0.46, 0.21]) performance. Unilateral resistance training exercises should be chosen for improving unilateral jumping performance, and bilateral resistance training exercises should be chosen for improving bilateral strength performance.


INTRODUCTION
Resistance training is one of the most widely used methods of enhancing athletic performance [1][2][3]. One of the challenging problems faced by practitioners is the issue of how to optimally choose the exercises to maximize training effects when prescribing resistance training programs. Typically, exercises in the weight room can be categorized into either unilateral or bilateral. A unilateral exercise is a weight bearing movement mainly or completely involving one limb (e.g. single leg squat, Bulgarian split squat and single leg jump), whereas, a bilateral exercise is a weight bearing movement executed evenly and simultaneously by both limbs (e.g. back squat, deadlift and countermovement jump). Traditionally, bilateral exercises are selected as the primary exercises for athletic development [4,5] due to their effects on improving strength and power [6][7][8][9]. In contrast,

Effects of unilateral vs. bilateral resistance training interventions on measures of strength, jump, linear and change of direction speed: a systematic review and meta-analysis
studies have found no differences between methods [25,31]. There is also confusion about which method is superior for the improvement of change of direction (COD) speed performance, with some favoring unilateral training methods [32] and others favoring bilateral methods [26]. Furthermore, there is also conflicting evidence on the effects of these training methods on bilateral strength, jump and speed performance [24-26, 28, 31, 32]. Recently, a meta-analysis carried by Moran et al., [33] concluded that there was no difference between the effect of unilateral and bilateral resistance training on horizontal movement speed (ES = 0.17, p = 0.30), but noted that the effect size was pooled by 7 short sprints, 2 CODs, 1 five alternated leg bounding and 1 stair climb outcomes from 11 included studies.
Given this review focused purely on the effects on horizontal speed, it is still unclear how both bilateral and unilateral exercises transfer to other key physical attributes, such as strength, jumping and COD speed. Therefore, the purpose of this systematic review and metaanalysis aimed to compare the effects of unilateral vs. bilateral resistance training on improving athletic performance. The hypothesis was that the effect of unilateral and bilateral resistance training would follow the principle of specificity. In other words, the unilateral resistance training would be better for improving the unilateral perfor-proposed that exercise should be as specific as possible to optimize the transfer of training. In addition, Bosch [14] proposed that intramuscular and inter-muscular coordination, outer movement resemblance and energy production are the key factors to evaluate and predict the specificity of the training methods. A compelling body of empirical evidence also supports exercise type specificity with regards to the range of motions, velocities, postures, and patterns [13,[15][16][17][18].
Actually, except for the obvious mechanical differences, unilateral and bilateral exercises also differ in intra-and inter-muscular aspects such as interhemispheric mutual activation [19], postural stability [20], relationship of force and velocity [21], psychological state [22] and lumbar load [23]. Based on those distinctions, unilateral and bilateral resistance training are expected not to transfer equally.
According to the training principle of specificity, unilateral resistance training should improve unilateral performance measures better compared with bilateral resistance training, and vice versa. However, current findings are conflicting with respect to which is better for the improvement of measures of athletic performance [24][25][26][27][28][29][30].
Some studies support the notion that unilateral resistance training (e.g. Bulgarian split squat) improves unilateral strength more than bilateral exercise training (e.g. back squat) [24,26], and some Effects of unilateral vs. bilateral training on performance mance, and the bilateral resistance training would be better for improving the bilateral performance.

Literature research
This systematic review was conducted under the Preferred Reporting Items for Systematic Reviews and Meta-Analysis Protocol (PRISMA).
One author searched the related articles from Web of Science  and PubMed (1949 to 2020). The following keywords inclusive of three main terms as unilateral, bilateral and performance were used and combined under Boolean's language with the operators AND and OR.

Inclusion Criteria
Studies were eligible if they met the following criteria: (1) implemented both unilateral and bilateral resistance training interventions; (2) the duration of training was longer than 4 weeks; (3) the training intensity was moderate to heavy; (4) included healthy participants aged from 16 to 40 years old in both genders; (5) had measured athletic performance (Speed, strength, COD, power test etc.) before and after the training intervention; (6) presented full data (mean and SD) that allowed effect sizes to be calculated; (7) the manuscripts were written in English and were published in a peer-reviewed journal.

Literature selection
The first author imported all records into Endnote software (X9. 3.3) and deleted any duplicates. Then, the first and fourth authors checked the title and abstract separately to exclude any unrelated articles, with the remaining full texts screened against the inclusion criteria.
There was no disagreement between the 2 authors on that aspect.

Risk of bias assessment
The fourth and fifth authors independently assessed the selected studies. In case of disagreement on certain item scores, the item scores would be given after discussion. Considering the most risk of bias assessment scales such as Delphi scale, PEDro scale and Cochrane scale are designed for medical research, studies about training interventions usually get very low score under these methodological scales [34]. We preferred the scale (Table 1) modified by Brughelli et al [34] and Hooren et al. [35]. This scale is deemed more suitable for sport science research, and includes 10 items, with each item rated as: 0 = clearly no/not reported, 1 = maybe, and 2 = clearly yes. The articles were rated poor with a total score lower than 10, moderate with a score between 10 and 15, good with a score > 15, and excellent with a score equal to 20.

Coding of the studies
The first author extracted the data from the selected literatures with a standard table. The code included: (1) participants: age, gender, identity and training experience; (2) interventions: frequency, duration, exercises, intensity and volume; (3) measurements and results: pre and post test outcome, with means and standard deviation.

Statistical analysis
The review manager software (5.3) was used for the meta-analytic comparison if more than one outcome were evaluated for a certain kind athletic performance measure between unilateral and bilateral resistance training (e.g. Strength, speed), subgroup analyses were performed.
Chi 2 and I 2 were calculated to test the heterogeneity. For I 2 values of 25, 50, and 75% represent low, medium, and high heterogeneity, respectively [36]. For Chi 2 with large value and p < 0.1 show evidences of heterogeneity. If p > 0.1 and I 2 < 50%, the fixed effects model was applied. Otherwise, the random effects model was applied and provoked further investigation through a subgroup analysis of moderator variables (Training experience, identity, gender, training frequency, training modalities, training weeks). In order to identify the presence of highly influential studies, a sensitivity analysis was executed by removing one study at a time. Studies were considered as influential if removal resulted in a change of heterogeneity (p) from significance (p < 0. 1) to non-significance (p > 0.1). into the meta-analytic data set and enlarge the type I error due to the same participants contributing to two or more effect sizes [39], Standardized mean differences (SMD) was calculated with the following algorithm [37]:  preference to the greater angle and shorter sprint distance.

Search results
The

Risk of bias assessment
In accordance with the modified scale [34,35], the scores of 14 included articles ranged from 12 to 18, the mean score was 15.
13 studies had allocated the participants in a random manner. Most of the studies got high score in item 4 and 7, but only 28.6% of the studies used a control group. 78.6% of the studies did not clearly describe the inclusion criteria and training experience of the participants (Table 3).
After dealing with the multiplicity, the final included outcomes for analysis were showed in table 2.

Quantitative analysis
The performance tests in included studies can be categorized into

Unilateral strength performance
After dealing with the multiplicity (

Bilateral strength performance
After dealing with the multiplicity, 9 outcomes were selected to do meta-analysis. The heterogeneity test was not statistically significant (Chi 2 = 6.77, p = 0.56; I 2 = 0%). In fixed effects model, overall effect was significant (p = 0.004). Pooled effect size and 95%CI were -0.43 (-0.71, -0.14) and classified as a small effect.

Unilateral jump performance
After dealing with the multiplicity, 6 outcomes were selected for meta-analysis. The original heterogeneity was high (I 2 = 60%), after alternated sensitive analysis, one study with eccentric overload training different from others was excluded, and the adjusted heterogeneity was low (Chi 2 = 4.85, p = 0.30, I 2 = 18%). In fixed effects model, the pooled effect size and 95% CI was 0.89 (0.52, 1.26) with statistical significance (p < 0.0001) and classified as a large effect.

Bilateral jump performance
After dealing with the multiplicity, 8 outcomes were selected for meta-analysis. The heterogeneity was not statistically significant

COD performance
After dealing with the multiplicity, 7 outcomes were selected for meta-analysis. The heterogeneity was not statistically significant

Speed performance
After dealing with the multiplicity, 6 outcomes were selected for meta-analysis. The heterogeneity was not statistically significant  [45]. There is evidence that unilateral and bilateral strength traning had a similar impact on muscle mass [46], girth [29] and cross-sectional area [47]. However, Helme et al., [48] found that almost 15% of the load was placed on the rear leg during the Bulgarian split squat. It could be inferred that the rear leg might be contributed to the concentric phase, which means that the lead leg might decrease force development. Anderson et al., [49] and McCurdy et al [50] found that the bilateral squat activated the kneejoint agonists (e.g., quadriceps) greater than the Bulgarian split squat, while the Bulgarian split squat showed greater antagonists such as the hamstrings, hip abductors and trunk musculature. Cumulatively, it could be speculated that bilateral strength training may produce greater knee agonist neuromuscular adaptation owing to higher load other than the muscle growth.
Four studies in this systematic review showed that the bilateral group mitigated the bilateral force deficit (BLD), but unilateral resistance training increased the BLD in measures of knee extension, isometric leg press and squat with one leg and both legs, respectively [24,27,29,43]. BLD is described as the sum of the maximum forces exerted by the left and right limbs unilaterally as being greater than the simultaneous exertion of both limbs bilaterally [51].
More recently, Bishop et al. found that a combination of bilateral and unilateral resistance training had superior effects on unilateral jump performance compared to bilateral. Consequently, it stands to reason that if greater improvements in unilateral jump performance are evident (compared to bilateral), it will have an effect on the BLD outcome (remembering that the BLD is a product of both unilateral and bilateral scores, presented as a single ratio number) [52]. In line with the definition and the findings by Bishop et al., the results of these 4 studies might be explained by the reason of specificity. Simply put, that unilateral resistance training is likely to enhance unilateral performance measures more than bilateral performance measures, and vice versa. Given the inherent differences in study design of these 4 studies (e.g.different levels of stability requirements in the chosen methods but still with similar effects on the BLD), it is speculated that neuromuscular factors induced by the mechanism of BLD may be more of a contributing factor to changes than stability.
The results of unilateral strength performance subgroup indicated that unilateral resistance training had a small effect (ES = 0.26) on improving unilateral strength. However, there were not statistically significant between-group differences. Our finding did not correspond well with included studies which found clear evidence that unilateral resistance training was better for improving unilateral strength performance [24,26]. The possible explanation was that these studies did not adjust each intervention group's mean changes between pre and post tests for analysis as in the current study, which may enlarge the probability to make the false inference due to the selection effect. For example, McCurdy et al., [31] found that 8 weeks' unilateral resistance training was more effective in improving 1RM of a Bulgarian split squat than the bilateral resistance training. However, after adjusting the pretest difference, both groups exhibited similar effects on the 1RM of Bulgarian split squat. In addition, the unilateral exercises may stimulate the stabilizing muscles in the core and knee to a greater extent than bilateral exercises [50,53], which are likely to be beneficial for improving stability and force transference through the kinetic chain. However, the greater agonist neuromuscular adapation of bilateral resistance training may counteract the superiority of stability and specificity of unilateral exercises in unilateral strength tests. Therefore, it is up to coaches to determine what the athlete needs and program accordingly.

Jump performance
The larger improvement in unilateral jump performance induced by unilateral rather than bilateral resistance training corresponded with the training principle of specificity. Five studies were included in the unilateral jump subgroups, all of which applied jumping exercises as intervention modalities [25,27,30,31,40] and found that the effect of unilateral jumping training were better (ES: 0.24-1.66) than bilateral jumping training. These findings might result from the differences in stability and neuromuscular adaptation of agonists. In unilateral jumping, the smaller supporting surface triggers a higher push relative higher load, which may result in greater range of motion and deeper squat depth for producing greater force, subsequently increase the contact time. Thus, it is conceivable that the unilateral jump group might be unable to adapt in the shorter exertion time to develop similar force as the bilateral jump group in the measure of bilateral jump test [58]. In contrast, bilateral jump exercises similar with the test modality may have a priority of familiarization and be adapted to exert force in shorter time.

Change of direction (COD) and speed performance
COD speed and sprint have been suggested to be largely unilateral exercises in previous research [12,59]. However, our results indicated that the effect size in the speed subgroup showed no difference between unilateral and bialteral resistance training, similar to recent findings [33]. However, COD was likely to be in favor of unilateral resistance training with a small effect.
Four of the 7 studies in the COD subgroup that adopted both the Bulgarian split squat versus back squat as the modalities of resistance intervention [24,25,28,32]  showed that bilateral exercise training had a larger effect in improving bilateral strength with statistical significance. Collectively, the co-activation in the stabilizing muscle groups (e.g. hamstrings), and helps maintain the head, arms, trunk and lower limbs in the same direction during both landing and taking off. As a result, the stability of the lower limbs are improved, and the absorption of the reaction force of the lower limbs decreases during the landing phase [54].
Bogdanis et al. [27]  The overall effect size of bilateral jumping subgroup was trivial in favor of bilateral exercises training, but the value was not statistically significant. The results may be the contribution of the mechanism of bilateral deficit and differences in kinetics [57]. When both legs contract simultaneously, the nerve activation of the left and right interhemisphere may be mutually inhibited, and there was a lack of bilateral activation while training unilaterally, so it would be expected that muscles would not achieve maximum voluntary contraction in unilateral jump group. Furthermore, the contact time for bilateral and unilateral jumps (CMJ) was 178-190 ms and > 250 ms respectively [21]. As above explained, the unilateral jump needs to improvement of bilateral strength may explain the trivial effect in favor of bilateral exercises training on improving speed.
It should be noted that the overall effect of each training method (unilateral vs bilateral) on COD and speed were not statistically significant. This could be attributed to the studies' small sample size, the lack of specificity of training methods with regards to the tests measures, and/or the low transference of jump and resistance training adaptations to sprint and COD performance measures [65].
However, Hopkins et al. [66] stated the smallest worthwhile enhancement (SWE) by 10% will help the athletes to win the game. SWE was calculated by the coefficient of the variablility (CV) of withinathlete's performance from competition to competition. With respect to short running events, it was found SWE was even lower to 0.3-0.5% [67]. This indicated that although the effect sizes were rather small or not significance between unilateral vs. bilateral resistance training, when inter-athlete variability was counted, the small difference in improving performance caused by the exercise selection were within a range that was meaningful for elite athletes. Thus, these findings still have significant implications for understanding how to choose unilateral or bilateral resistance exercise for optimizing speed performance.
It should be acknowleged that some limitations exist in our paper.
Firstly, the lower number (< 10) of studies in subgroup analyses means it was not possible to do further meta-regression analysis.
Thus, as is often the case in science, more research is likely needed to provide greater clarity between the two training methods.